The technology of drilling oil and gas wells has evolved, and is evolving, in response to a variety of stimuli. It has manifestly become important to conduct drilling operations in a manner which is not substantially damaging to the environment. One aspect of this is to handle and dispose of spent drilling fluid and drilled solids.
In a conventional well drilled with a conventional water based mud, the spent drilling fluid and drilled solids accumulate in one or more pits near the drilling rig. Typically, after drilling operations are complete and the rig moved away, the earthen dike on the reserve pit is broken and the spent drilling fluid spread in the pasture where the well was drilled. The shale pit, containing largely drilled solids, is simply covered up. This technique was widely used, and is still widely used, in many areas.
There are some situations where it is necessary or desirable to dispose of drilled solids and spent drilling fluid in a more formal manner. For example, farmers often require drilled solids and spent drilling fluid to be hauled from the site so as not to diminish the agricultural productivity of the land. In offshore wells, or wells drilled in or near inland bays, fresh water lakes, rivers and the like, it is often necessary to haul drilled solids and spent drilling fluid from the site. Other situations, such as wells drilled near cities, require disposal of drilled solids and spent drilling fluid.
The original technique was to run the spent drilling fluid and drilled solids through a cyclone type separator or centrifuge providing one outlet stream having a high proportion of solids and another outlet stream having a high proportion of liquid. The volume of the solids, although high, is small compared to the volume of liquid. The liquid is mostly water but contains so much fine grained material or silt that it cannot be handled or treated like water. In other words, the liquid discharge from a cyclone separator still contains so much solids that it is disposed of as if it were a troublesome material.
In response to this dilemma, attempts have been made to add flocculants to drilling mud, spent drilling mud and the liquid discharge from running drilling mud or spent drilling mud through a cyclone separator. The concept is to flocculate the solids in the liquid to create larger sized particles that will drop out of suspension or which can readily be separated to leave a substantial volume of water that can be treated as simply water. The concept of adding a flocculant to a drilling mud for the purpose of dropping solids out of suspension is known as shown in U.S. Pat. Nos. 3,737,037; 4,127,482; 4,234,421; 4,353,803; 4,457,842; 4,913,585 and 5,093,008. It is this type approach that this invention most nearly relates.
The prior art approach is to add a flocculant to the drilling mud or to mix flowing streams of flocculant and drilling mud. The attempt has been to minimize the volume of the flocculant solution because a very dilute flocculant adds more water and thus more volume to the material being separated thereby compounding the disposal of the liquid fraction.
There are numerous difficulties with the prior art approaches. The prior applications are not effective when a drilling fluid is dispersed with lignosulfonates and other thinners or dispersants.
As the percentage of solids increases, the internal addition of flocculants into drilling mud tends to increase its viscosity. This makes separation of the liquid and solid phases much more difficult. One example of a very high solids drilling mud is the discharge from a mud cleaner, cyclone or centrifugal separator.
The same problem occurs with liquids that are highly viscous before the addition of flocculant. The internal addition of flocculant to a rather viscous drilling mud makes the mud even more viscous. As in the case of high solids drilling muds, this makes separation of the liquid and solid phases much more difficult.